I think the use of the car is going to direct you.
The S2000 is a better track car. If you're idea of fun is going to the track every weekend, this is the car for you. However, it's not all that great of a convertible, it's very highly strung to drive, and the interior is uncomfortable and looks like it came from a $14,000 Civic (if that).
I would pick a new MX-5 over the previous-gen MSM. It's just so much better in every way (including fixing the accomodations issue). A total joy to drive, it is also an exceptional convertible design with good lockable storage, a decent size trunk for this class, and a top that is a work of art. Yet, it can dance at the track meet too. The Miatas also have enjoyed exceptional reliability, even better than the S2000 in several surveys.
The Solstice/SKY is your typical GM screw-up of a car. They may have gotten the powertrain down, but the convertible aspect is about the worst I have seen in a modern car. It is impossible to take this car for a top-down weekend cruise because you have no trunk when the roof is lowered. The interior is cheap, flimsy, and easily broken (as I have seen on multiple samples), and lockable storage is at a premium. Either the MX-5 or the S2000 is a far better choice. Even a used MR2 is better, as terrible of a convertible as that is.
Between the two you selected, I would pick the Mazdaspeed Miata, as it is a better all-around convertible toy than the S2000, which I pick if you're looking for primarily a track car.
However, you really owe it to yourself to check out the new MX-5. It transcends the previous car, as good as it was.
